When making advanced semiconductor devices, copper interconnects may offer a number of advantages over those made from aluminum. For that reason, copper has become the material of choice for making such devices' interconnects. As device dimensions shrink so does conductor width—leading to higher resistance and current density. Increasing current density can increase the rate at which copper atoms are displaced when current passes through a copper conductor. Such electromigration can cause accumulation of vacancies, which may lead to voids. If the voids grow to a size that creates metal separation, they may cause an open-circuit failure.
One way to prevent electromigration from causing interconnect failure is to limit the amount of current that passes through the conductor. That solution to the electromigration problem is impractical, however, because devices will operate at progressively higher currents, even as they continue to shrink. As an alternative, interconnect reliability can be enhanced by doping the interconnect—as adding dopants to the conductor can reduce the speed at which copper diffuses. When doping a copper interconnect to reduce electromigration, current processes dope the entire conductor. Techniques for achieving such global doping include using: doped seed layers, a plating or sputtering process that adds dopants to the conductor as it is formed, or ion implantation to implant dopants into the conductor.
Doping the entire interconnect can raise its resistance significantly. To reduce RC delay that is associated with high resistance, it may be necessary to limit dopant concentration—or dispense with doping altogether, e.g., when forming high speed conductors. When low level doping is required to limit conductor resistance, the electromigration mitigating impact that such doping provides is reduced.
Accordingly, there is a need for an improved process for making a semiconductor device that includes copper interconnects. There is a need for such a process that reduces electromigration without significantly raising conductor resistance. The method of the present invention provides such a process.